Currently, hydrostatic drives are used for propelling a wide variety of machines including, but not limited to, small tractors, combines, and skid steer loaders, under low and low-to-mid-range travel speed conditions. Advantages of the hydrostatic drives include efficiency at low speeds and ability to infinitely vary speed within the speed range thereof under load without clutching. However, hydrostatic drives have been found to be inefficient at mid-to-higher travel speeds, such as wherein a machine is to be driven over a road to or from a job site, and when used under such conditions, operation is often strained and travel speed is too slow.
As an alternative for mid-to-higher travel speeds, including road travel, or "roading" as it is known, multiple speed geared transmissions are preferred, as they provide excellent speed ranges. However, the need for clutching to disengage the engine when shifting, has been found to result in jerky operation and stalling at low speeds when under load.
For some machines, particularly small wheel loaders, back hoes and combinations thereof, a multiple speed geared transmission is coupled with a low ratio torque converter to provide both a limited roading capability and better low speed operability, but this has been found to provide less than desirable efficiency under the low speed conditions. Other machines have used hydro-mechanical transmissions including a multiple speed gear box in combination with a single speed range hydrostatic drive, but the single speed range hydrostatic drive provides only limited operability and efficiency.
Therefore, what is required is a transmission system providing the advantages of hydrostatic drive and direct drive which enables avoiding the attendant shortcomings and inefficiencies of the known transmission systems.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.